In recent years, semiconductor devices which use an amorphous-oxide-based semiconductor thin film have been receiving attention. Such a semiconductor thin film can be formed at low temperature, and has a large optical band gap and is optically transparent to visible light. Hence, a flexible transparent thin film transistor (TFT) or the like can be formed on a plastic substrate, a film substrate, or the like.
As an example, Japanese Patent Application Laid-Open No. 2002-76356 discloses a technology with regard to a TFT in which an oxide film mainly formed of Zn—O is used as an active layer.
“Nature”, Vol. 432, 25, November 2004 (pp. 488-492) discloses a technology with regard to a TFT in which a non-crystalline oxide film formed at room temperature and containing indium, zinc, and gallium is used as an active layer. The S value of the TFT is about 2 V/decade, which is relatively large, but the field-effect mobility of the TFT is as high as 6 to 9 cm2/Vs, and thus, its application to an active matrix system which is desired to be used in a flat panel display apparatus using liquid crystal, electroluminescence, or the like is expected.
“Journal of Non-Crystalline Solids” 352 (2006) 2311 discloses that an oxide thin film formed at room temperature and mainly formed of indium oxide is used as a channel layer (active layer) of a TFT. Depending on the material of a gate insulating film, the field-effect mobility of the TFT is 10 to 140 cm2/Vs and the S value of the TFT is 0.09 to 5.6 V/decade.
United States Patent Application Publication No. 2006/0108636 discloses a technology with regard to addition of an impurity such as Li, Na, Mn, Ni, Pd, Cu, Cd, C, N, or P to an active layer of a TFT in which an amorphous oxide film formed at room temperature and containing indium, zinc, and gallium is used as an active layer. In this way, by adding an impurity element in the active layer, the carrier density is controlled, and as a result, a TFT with a large current on/off ratio is thought to be obtained.
However, according to findings by the inventors of the present invention, the TFT disclosed in “Nature”, Vol. 432, 25, November 2004 (pp. 488-492) has a problem that, depending on the atomic composition ratios of the metallic elements mainly forming the non-crystalline oxide film used as the active layer, the TFT characteristics vary greatly.
With regard to the TFT disclosed in United States Patent Application Publication No. 2006/0108636, the effect of adding an impurity when the atomic composition ratios of the metallic elements mainly forming the active layer are changed is not clarified.
With regard to “Journal of Non-Crystalline Solids” 352 (2006) 2311, according to findings by the inventors of the present invention, the environmental stability of the In—O film formed at room temperature is low, and the resistivity of the In—O film varies greatly when the In—O film is left in an atmosphere. For example, when the film is left in an atmosphere at a temperature of 20° C. and at a humidity of 50% for a month, the resistivity is observed to decrease by one or two orders of magnitude. Further, the above-mentioned decrease in resistivity is similarly observed in the oxide semiconductor disclosed in Japanese Patent Application Laid-Open No. 2002-76356 mainly formed of Zn—O.